1. Field of the Invention
The invention relates to the field of vibrators for transporting articles across an article-carrying surface by producing an asymmetrically timed vibratory motion of the surface relative to a fixed base. A piston/cylinder arrangement and an associated pneumatic valve assembly move a sliding carriage coupled to the article-carrying surface such that the sliding carriage accelerates relatively more quickly when moving in one direction and more slowly when moving in the other direction.
Preferably, two pistons are aligned on a common axis and have power strokes in opposite directions from respective expansion chambers at their inside ends. The pneumatic valve assembly includes a pair of spaced cam rods and a crossbar extending between the cam rods at about their midpoints. The cam rods are mutually parallel with the axis of the pistons, slide in unison with the carriage, and cooperatively push the crossbar between opposite extreme positions. The crossbar controls the air flow for the power and exhaust strokes of each piston, at half cycles that are out of phase from each other. The half cycles are differently timed by a difference in the air discharges from the respective pistons. Due to inertia and friction, articles on the article-carrying surface are more readily displaced relative to the fixed base in the direction in which the carriage is more slowly accelerated, because friction is more likely then to hold the articles on the accelerating surface notwithstanding their inertia.
2. Prior Art
Vibrators are known in the art and have article-carrying surfaces that vibrate rapidly in back and forth strokes. Vibrators are useful for moving articles, such as screws, from areas that are inaccessible or for moving articles that are inconvenient to move by other means. For example, a typical application is to remove material from under a machine or a punch press. Such vibrators can operate on compressed air, i.e., without electrical connections, permitting their use in areas where electrical currents and/or voltages are to be avoided.
Springs are not preferred for vibrators, which advantageously operate for long periods of time over many many cycles, often at high cyclic-rates. Under such conditions, springs are subject to degradation and failure, and consequently are undesirable. A pneumatic arrangement with an oppositely displaceable piston/cylinder arrangement, however, is durable and long lived.
An exemplary vibrator of this type is disclosed, for example, in Johnson U.S. Pat. No. 4,593,603 . This vibrator has a base with opposite ends having piston bores or chambers. Pistons are disposed in the piston chambers and extend in opposite directions. A carriage overlays the base and has flanges on opposite edges fastened to the outer ends of the pistons. The carriage and pistons move in unison relative to the base.
In Johnson the pistons are of different diameters. Both pistons carry an O-ring at the middle. For each piston, an expansion chamber is defined between the closed end of the piston bore and the O-ring. The larger diameter piston has a threaded aperture in its inner end (i.e., its head), a valve mechanism is coupled to the threaded aperture and disposed entirely inside the expansion chamber of the larger diameter piston bore. The valve mechanism controls the intake and exhaust of compressed air for the two expansion chambers.
The action of the valve mechanism causes the two pistons to reciprocate. The expansion chamber of the smaller diameter piston bore is supplied compressed air through a relatively smaller hole, while the expansion chamber of the larger diameter piston bore is supplied air through a relatively larger hole. Acceleration of the larger diameter piston from the retracted to the extended position therefor is faster than the acceleration of the smaller diameter piston. Thus the pistons accelerate the carriage asymmetrically. Consequently, the asymmetrically-accelerated article-carrying surface of the carriage transports articles in one direction.
More particularly, during the slower acceleration in one direction, friction between the articles and the article-carrying surface tends to better retain the articles on the surface as the surface moves. The articles accelerate together with the surface and are displaced relative to the base by an incremental distance during the stroke. During the faster acceleration in the opposite direction, the inertia of the articles is more likely to overcome their frictional connection with the surface, in which case the accelerating article-carrying surface moves beneath the relatively stationary articles. The overall effect is that the articles are transported in the same direction as the slow acceleration stroke in short repetitive increments. The operating principles of vibrators and the exemplary vibrator above are disclosed more fully in Johnson U.S. Patent No. 4,593,603 , which is hereby incorporated in its entirety.
It would be advantageous if certain improvements could be made on the known vibrator as described. A shorter and more compact carriage arrangement could be provided if the valve mechanism could be moved out of the expansion chamber. Also, improvements could be made by providing a means to adjust the ratio of fast acceleration to slow acceleration so as to achieve the most effective movement to obtain article motion. These and other aspects are addressed according to the present invention.